1. Field of the Invention
The present invention relates to an image reading device, an image reading apparatus, and a method therefor and, more particularly, to an image reading device used in an image scanner or facsimile apparatus for reading light from an original, an image reading apparatus, and a method therefor.
2. Description of Related Art
As an image reading apparatus used in an image scanner or facsimile apparatus, an apparatus using a reduction optical system shown in FIG. 21 is known. Referring to FIG. 21, light is irradiated from a light source 5 to an original 6. The light reflected by the original 6 is deflected through 90.degree. by a mirror 4 and focused on a CCD image sensor 1 through a lens 2, thereby converting the light into an image signal. A correction plate (shading correction plate) 3 for correcting a power of cos4 of a white output waveform lens. The image signal output from the CCD image sensor 1 is A/D-converted into digital data by an analog-to-digital (A/D) converter incorporated in an IC (not shown). The digital data is subjected to an image process. An inexpensive xenon (Xe) lamp or LED is often used as a light source for this image reading apparatus.
A printer serving as an image reading apparatus is available in which a scanner unit is mounted in place of a print head unit including an ink tank detachably mounted in the printer.
The above techniques pose the following problems. Since the absolute light amount of the Xe lamp or LED used in the image reading apparatus is small, a charge accumulation time (to be simply referred to also as an accumulation time hereinafter) in reading operation in the CCD image sensor 1 must be prolonged, and the sensitivity of a reception system must be improved in order to obtain stable image information. The sensitivity characteristics of the light-receiving portion for photoelectrically converting light reflected by the original have limitations although high-speed driving of a CCD device serving as a light-receiving element has advanced. In order to obtain a sufficiently high S/N ratio, the original must be illuminated in a sufficiently large light amount.
For this purpose, to increase the light amount by increasing a drive current to the LED, the drive current changes due to the wavelength of emission light upon heating of the LED itself and a change in forward voltage. As a result, a stable emission intensity cannot be obtained.
In a printer incorporated in a notebook personal computer, when a scanner unit is mounted in place of a print head unit, the scanner unit must be made compact and simple because the printer of this personal computer is compact. In addition, a reduction in power consumption is desired in consideration of driving the notebook personal computer using a secondary battery.
To correctly set the dynamic range in reading an image, a white reference value and an offset value must be measured for each scanner unit because the respective scanner units vary in characteristics. However, in mounting a scanner unit in the built-in printer of the notebook personal computer, white reference setting for setting the white reference value and the offset value is often omitted due to downsizing and low cost. In this case, every time the white reference value and the offset value are to be set, a user must feed originals for measuring the white reference value and the offset value to allow them to read these values, and must set the white reference value and the offset value.